Embodiments of the invention relate to systems and methods for installing a topside or deck on a substructure to form a fixed or floating offshore platform. More particularly, embodiments of the invention relate to a novel system and method for releasing a barge from the topside after float-over installation of the topside on the substructure.
Float-over installations offer opportunities to install heavy topsides beyond the lifting capacity of available crane vessels on offshore substructures located in remote areas. A float-over installation includes four primary procedures. The first procedure involves transporting the topside or deck to the offshore substructure. Typically, the topside is placed on a barge or heavy transport vessel and towed to the substructure.
The second procedure involves docking the transport barge to the installed substructure. The barge is maneuvered into the slot of the substructure, such that the topside is floated over and substantially aligned with the substructure. Once in the slot, mooring lines, sometimes in combination with a fendering system, are utilized to suppress surge and sway motions of the barge. After the mooring lines are set, deballasting of the substructure commences.
The third procedure involves transferring the load of the topside from the barge to the substructure, and is a critical phase of the float-over installation. Deballasting of the substructure continues as the substructure rises toward the topside. Once the topside and the substructure reach close proximity, the two bodies may impact each other repeatedly due to wave action. Such impacts may damage the structures when the relative motion between the two bodies is not controlled. As deballasting of the substructure continues, the weight of the topside is gradually transferred from the barge to the substructure. After a critical fraction of the weight is transferred, the relative motion between the two bodies ceases. At that point, the two structures move as a single unit, and the possibility of damage due to hard impact is eliminated. Therefore, it is desirable to complete the load transfer up to the critical fraction as quickly as possible.
After the topside is fully supported by the substructure, the legs of the two structures are coupled by welding legs extending downward from the topside to legs extending upward from the substructure. To achieve the high quality welds required to withstand the harsh load regimes of offshore environments, proper alignment of the topside with the substructure during the float-over operation is critical.
The final procedure involves separating the barge from the topside, and is also a critical phase of the float-over installation. The substructure is deballasted further until the topside separates from the barge. At and immediately after separation, the relative motions between barge and topside pose a danger of damage due to impact between these bodies. That danger can be minimized by rapid separation of the barge and the topside. To promote such rapid separation, the topside may be supported on the barge by a number of loadout shoes. At the appropriate time, the loadout shoes are actuated to quickly collapse or retract, thereby providing rapid separation between the barge and the topside. These systems, however, have a propensity to malfunction and permit hard contact between the loadout shoes and the topside. In any event, hard contact between the barge and the topside may continue until the substructure is deballasted to provide sufficient separation between the barge and the topside. After which point, the barge is towed from the installation site.
Thus, embodiments of the invention are directed to apparatus and methods that seek to overcome these and other limitations of the prior art.